The term seems to be used with either a symmetrical or asymmetrical configuration. The only difference seems to be that you do not use a pole (symmetrical pole/sprint pole) and it joisted out of a spinnaker sock? It seems to be the way to go when you are sailing short-handed.

Not sure but I believe the Asymmetrical (A sail) is cut differently (draft) than a Symmetrical. I think the A-sail is used when the apparent wind is between 90-120 while the Symmetrical (conventional) is used when the wind is further aft (> 120 apparent).

A symmetrical spinnaker has two clews both at the same height so that the luff and leech are interchangable. While asymetrical only has one clew and the luff is longer than the leech so the two are not interchangable.